The present invention is a high-consistency organosiloxane composition comprising fluorinated and non-fluorinated polyorganosiloxanes, which upon the addition of suitable cure components can be cured to form elastomers having improved tear strength and good resistance to hydrocarbon oil weep. The present composition comprises a reinforcing silica filler that has been treated with a tetraalkyldisilazane.
Curable compositions containing mixtures of fluorinated and non-fluorinated high consistency polydiorganosiloxanes are known. One of the more common repeating units that constitutes at least a portion of the units present in fluorinated polydiorganosiloxanes is the methyl-3,3,3-trifluoropropylsiloxy unit. When compared with cured elastomers prepared from polydimethylsiloxanes, elastomers prepared from fluorinated polydiorganosiloxanes typically exhibit inferior physical properties such as tensile and tear strength and elasticity and have a higher solubility in polar organic liquids such as alcohols, ketones and esters. This reduction in properties is compensated for by a lower solubility and a higher resistance to degradation in the aliphatic hydrocarbons present in aviation and automotive oils and fuels.
Compositions comprising high-consistency fluorinated and non-fluorinated polydiorganosiloxanes when cured typically provide an elastomer with physical properties that represent a compromise, with intermediate values for tensile, tear, and solvent resistance. One approach to increasing the physical properties of compositions comprising fluorinated and non-fluorinated polydiorganosiloxanes when cured, is to increase the compatibility of the polymers. For example, Evans et al., U.S. Pat. No. 4,960,811, describe a composition where 100 parts of a modified fluorinated polydiorganosiloxane containing both vinyl and silanol groups is blended with from one to 10 parts of a polydimethylsiloxane gum containing from one to about 10 weight percent of vinyl radicals.
Maxson, U.S. Pat. No. 5,302,632, describes a process for increasing the compatibility between high-consistency fluorinated and non-fluorinated polydiorganosiloxanes with a resultant increase in the physical properties of elastomers prepared from mixtures of these polymers. The improved compatibility is achieved by blending the polymers together with a reinforcing silica filler that is reacted with both fluorinated and non-fluorinated liquid polydiorganosiloxanes as filler treating agents. The high consistency polydiorganosiloxanes contain at least two alkenyl radicals per molecule and are cured by a hydrosilation reaction.
The present inventor has found that the tear strength of compositions comprising a high-consistency fluorinated and non-fluorinated polydiorganosiloxane can be improved by treating the reinforcing silica filler used in such compositions with a tetraalkyldisilazane. The improved tear strength can be achieved while maintaining a good resistance to hydrocarbon oil weep.